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UNIVERSITY  OF  ILLINOIS 

Agricultural  Experiment  Station 


BULLETIN  No.  304 


A  STUDY  OF  FACTORS  AFFECTING  THE 

EFFICIENCY  AND  DESIGN  OF 

FARM  SEPTIC  TANKS 

BY  E.  W.  LEHMANN,  R.  C.  KELLEHER,  AND  A.  M.  BUSWELL 

A  joint  publication  by  the  University  of 
Illinois  Agricultural  Experiment  Station 
and  the  Illinois  State  Water  Survey 


URBANA,  ILLINOIS,  APRIL,  1928 


CONTENTS 

PAGE 

I— STUDY  OF  SEWAGE  FLOW  FROM  FARM  HOMES 300 

Method  of  Measuring  Flow 300 

Results  Obtained 301 

Conclusions 302 

II— STUDY  OF   SINGLE-  AND   MULTIPLE-CHAMBER   SEPTIC 

TANKS:   EFFECT  OF  RETENTION  PERIOD  ON  EFFLUENT  303 

Description  of  Experimental  Tanks 303 

Dosing  Tanks  with  City  Sewage 305 

Flow  of  Sewage  to  Farm  Tanks 307 

Collection  of  Samples 307 

Analytical    Methods 309 

Measurement  of  Scum  and  Sludge 309 

Temperature    Records 310 

Duration  and  Conditions  of  Tests 310 

Results  Obtained 315 

Discussion   and    Conclusions 315 

III— STUDY  OF  TWO-CHAMBER  SEPTIC  TANKS  HAVING  DIFFER- 
ENTLY SHAPED   CROSS-SECTIONS 321 

Description  of  Experimental  Tanks 321 

Dosing  Tanks  with  City  Sewage 321 

Flow  of  Sewage  to  Farm  Tanks 321 

Collecting  and  Analyzing  Samples 322 

Measurements  of  Scum  and  Sludge 323 

Duration  and  Conditions  of  Test 323 

Results  Obtained 327 

Discussion   and   Conclusions 330 

IV— RECOMMENDATIONS  FOR  THE  DESIGN  OF  SIMPLE  FARM 

SEPTIC  TANKS. 332 

APPENDIX..  .  335 


This  report  on  "A  Study  of  Factors  Affecting  the  Efficiency  and  Design  of  Farm  Septic  Tanks" 
is  printed  also  as  Bulletin  27  of  the  Illinois  State  Water  Survey 


A  STUDY  OF  FACTORS  AFFECTING  THE 

EFFICIENCY  AND  DESIGN  OF 

FARM  SEPTIC  TANKS 

BY  E.  W.  LEHMANN,  R.  C.  KELLEHER,  AND  A.  M.  BUSWELL* 

With  the  introduction  of  modern  plumbing  into  the  farm  home  a 
demand  for  a  simple  and  effective  means  of  sewage  disposal  on  the 
farm  was  created.  The  septic  tank  was  found  best  suited  to  this  dis- 
posal problem,  and  a  number  of  designs  of  small  tanks  were  developed 
by  various  agencies,  many  of  them  evolved  by  more  or  less  "cut-and- 
try"  methods.  Because  of  poor  design  many  of  the  tanks  failed  to 
function  properly,  and  many  others  were  more  complicated  and  more 
expensive  than  necessary. 

Several  investigations  concerning  septic  tanks  have  been  con- 
ducted by  the  experiment  stations  connected  with  the  state  univer- 
sities, but  up  to  1922  there  was  a  lack  of  fundamental  data  on  the 
factors  affecting  the  design  of  simple  farm  septic  tanks.  In  1922  the 
Illinois  Station,  in  cooperation  with  the  Illinois  State  Water  Survey, 
began  a  study  of  tanks  of  simple  rectangular  design  which  could  be 
easily  constructed  by  inexperienced  workmen.  The  investigation  was 
continued  for  five  years,  and  during  this  time  more  than  1,100  chemi- 
cal analyses  were  made  of  effluent  from  experimental  tanks. 

The  purpose  of  this  investigation  was  to  study:  (1)  the  amount 
and  rate  of  sewage  flow  that  a  farm  septic  tank  may  be  expected  to 
care  for;  (2)  the  effect  of  the  size  of  the  tank  on  its  efficiency  for  a 
given  amount  of  sewage ;  (3)  the  relation  of  length,  width,  and  depth 
of  tank  to  efficient  operation;  (4)  the  relative  efficiency  of  single- 
chamber  and  multiple-chamber  tanks. 

The  results  of  this  study  have  led  to  the  following  conclusions 
which  will  be  found  further  elaborated  in  the  following  pages: 

1.  Inasmuch  as  the  flow  of  sewage  per  person  from  farm  homes  is 
subject  to  wide  variation,  the  tank  should  be  so  designed  as  to  make 
an  average  allowance  for  sewage  flow  of  18  to  25  gallons  per  person 
per  day  depending  upon  the  size  of  the  family  (page  332). 

2.  Ordinarily  it  is  not  practical  to  build  a  tank  smaller  than  the 
size  required  for  seven  people. 


1E.  W.  LEHMANN,  Chief  in  Farm  Mechanics;  R.  C.  KELLEHER,  formerly  First  Assistant  in 
Farm  Mechanics;  and  A.  M.  BUSWELL,  Chief,  Illinois  State  Water  Survey.  The  authors  wish  to 
express  to  Mr.  Harold  E.  Babbitt,  Professor  of  Municipal  and  Sanitary  Engineering,  University 
of  Illinois,  and  to  Mr.  Harry  F.  Ferguson,  Chief  Sanitary  Engineer,  Illinois  State  Department  of 
Public  Health,  appreciation  for  valuable  suggestions  on  the  interpretation  of  data.  Thanks  are 
extended  also  to  Mr.  F.  P.  Hanson,  formerly  Extension  Specialist  in  Farm  Mechanics,  University 
of  Illinois,  and  to  Mr.  A.  A.  Brensky,  formerly  Assistant  Engineer,  Illinois  State  Water  Survey, 
for  suggestions  and  cooperation  in  the  erection  of  the  experimental  plant. 

299 


300 


BULLETIN  No.  304 


[April, 


3.  In  a  single-chamber  tank  a  72-hour  retention  period  should  be 
provided  (Fig.  24) . 

4.  In  a  two-chamber  tank  a  72-hour  retention  period  should  be 
provided  in  the  first  chamber  and  an  additional  retention  period  of  36 
hours  in  the  second  chamber  (capacities  being  in  the  ratio  of  2  to  1, 
or  a  total  retention  period  of  108  hours)  (Fig.  25). 

5.  When  properly  designed  the  two-chamber  tank  is  more  efficient 
than  the  one-chamber  tank,  particularly  if  the  two-chamber  tank  is 
provided  with  50  percent  larger  capacity,  as  recommended  above. 


I— STUDY  OF  SEWAGE  FLOW  FROM  FARM  HOMES 
Method  of  Measuring  Flow 

The  first  step  in  this  study  was  to  determine  the  amount  and  rate 
of  sewage  flow  that  a  farm  septic  tank  may  be  expected  to  care  for. 

A  tipping-bucket  meter  was  constructed  and  installed  at  a  home 
on  the  University  farm  occupied  by  three  people  (Figs.  1  and  2) .  The 


FIG.  1. — TIPPING-BUCKET  METER  FOR  DETERMINING  THE 
QUANTITY  AND  RATE  OF  SEWAGE  FLOW  FROM  A 

FARM  HOME 

The  meter  above  is  shown  in  the  laboratory  being 
calibrated. 

home  was  supplied  with  University  water  pressure,  and  the  tenant  was 
not  charged  for  the  water  used.  The  sewer  connections  consisted  of  a 
toilet,  a  kitchen  sink,  a  bathtub,  and  a  laundry  drain. 

The  meter  was  constructed  of  sheet  copper  and  reinforced  with 
strap  iron.  It  was  fitted  with  an  electrical  contact  brush  for  oper- 
ating the  time  recorder,  so  that  each  tipping  of  the  bucket  closed  the 
circuit  and  operated  the  recording  pen.  The  tape  chart  on  the  time 
recorder  had  a  paper  travel  of  6  inches  an  hour.  The  amount  of  dis- 
charge per  dump  was  adjusted  by  changing  the  position  of  the  counter- 
weight shown  at  the  right  end  of  the  tipping-bucket  in  Fig.  1. 


1928] 


EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS 


301 


The  tipping-bucket  was  first  calibrated  while  operating  in  the 
laboratory.  The  laboratory  calibration  was  not  satisfactory,  since  the 
conditions  were  somewhat  different  under  actual  operation.  The  fric- 
tion on  the  bearings  was  different  while  operating  in  the  manhole,  and 
a  thin  film  of  organic  matter  covered  the  inside  of  the  bucket  after  a 
week  or  ten  days  of  operation.  The  bucket  was  therefore  recalibrated 
in  the  manhole  after  the  formation  of  film  had  apparently  become  con- 
stant. A  water  meter,  installed  in  the  home  and  used  for  this  calibra- 
tion, indicated  that  the  bucket  was  discharging  1%  gallons  each  time 
it  tipped. 

]WO     WIRt-S    TO    E.ULCTR1C  TlME. 


|WO    WIRE.9    TO    LULCTR1C    IIME.  — "~1       J 

RtcoRDtt).  THI.TIME.  RECORDER  is  Vw 

MOUNTED    ON    A     POST    NEAR  7HE.\ 


FIG.  2. 


THE  TIPPING-BUCKET  METER  SHOWN  IN  FIG.  1  INSTALLED  IN  A  MANHOLE 
AT  THE  OUTLET  END  OF  THE  SEPTIC  TANK 


The  quantity  and  rate  of  flow  of  sewage  were  measured  at  the 
septic  tank  by  means  of  the  tipping-bucket  meter.  The  water  con- 
sumption was  measured  with  an  ordinary  water  meter  while  the  sew- 
age measurements  were  being  made. 

Results  Obtained 

The  average  hourly  rate  of  flow  of  sewage  from  this  farm  home 
over  a  period  of  14  days,  as  recorded  by  the  tipping-bucket  meter,  is 
shown  in  Fig.  3.  The  average  flow  over  the  entire  period  was  1.42  gal- 
lons per  capita  per  hour,  or  34.1  gallons  per  capita  per  day.  The  aver- 
age water  consumption  at  this  home  during  the  time  the  sewage 
measurements  were  taken  was  39.5  gallons  per  capita  per  day.  Part 
of  the  discrepancy  between  the  water  consumption  and  the  sewage 
flow  was  due  to  the  fact  that  some  water  was  used  for  watering 
poultry. 

The  results  of  this  study  indicate  that  in  general  the  water  con- 
sumption in  a  home  is  an  approximate  index  of  the  sewage  flow.  Ado^i- 
tional  measurements  of  water  consumption  were  then  made  with  water 
meters  at  eight  other  farm  homes.  These,  as  well  as  the  first  home, 


302 


BULLETIN  No.  304 


[April, 


were  supplied  with  water  under  pressure  and  were  equipped  with 
plumbing  fixtures  and  a  sewage-disposal  system.  Each  had  a  kitchen 
sink,  a  bathtub,  a  lavatory,  a  toilet,  and  laundry  equipment,  with  the 
exception  of  Farms  7  and  8,  which  had  no  lavatory.  Farms  1  to  6 
were  equipped  with  home  water-pressure  systems.  Farms  7,  8,  and  9 
were  supplied  with  University  water  pressure  and  the  tenants  were 
not  charged  for  the  water  used.  The  data  for  all  nine  homes  are  given 


^ 

PI 

£3 

5 


S  PEQ  CA 

f\j 


/£  / 


^ 


A 


Z  3  4-  5  6  7  &  3  10  II  IZ  I   2   3  4-  5  6   7  Q  3  10  II  12. 
A.M.  KM 

floue  OF  DAY 


FIG.  3.  —  RATE  OF  SEWAGE  FLOW  FROM  A  FARM  HOME 

OF  THREE  PEOPLE 

The  curve  shows  the  average  hourly  rate  of  flow  over  a  14-day 
period  during  April  and  May.  The  maximum  flow  during  one  hour 
was  35  gallons,  or  11.7  gallons  per  capita  per  hour;  this  large  flow 
occurred  one  Saturday  between  7:00  and  8:00  p.  m.  The  shape  of 
the  curve  will  of  course  vary  from  home  to  home,  depending  upon 
the  habits  of  the  occupants. 

in  Table  1.  It  will  be  noted  that  the  consumption  on  Farms  7,  8,  and 
9  was  higher  than  the  average  consumption  on  the  other  six  farms. 
The  measurements  indicated  that  in  general  the  water  consump- 
tion was  greater  during  the  summer  than  during  the  winter. 

Conclusions 

This  study  leads  to  the  following  conclusions: 

1.  In  farm  homes  where  all  of  the  house  supply  is  used  in  the 
home,  and  where  all  the  house  drainage  is  discharged  into  the  sewer 
line,  the  sewage  flow  is  approximately  equivalent  to  the  water  con- 
sumption. 

2.  The  rate  of  sewage  flow  varies  considerably  for  different  hours 
of  the  day. 

3.  The  monthly  variations  in  sewage  flow  depend  to  a  large  extent 
upon  the  monthly  variation  in  water  consumption.   The  higher  tem- 
peratures during  the  summer  months  tend  to  increase  water  con- 
sumption and  sewage  flow.  However,  both  hourly  and  monthly  varia- 
tions of  flow  are  affected  greatly  by  the  habits  of  the  people  and  by 
local  conditions. 


EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS 


303 


TABLE   1. — WATER   CONSUMPTION    AT  FARM   HOMES   EQUIPPED   WITH    MODERN 

PLUMBING 


Farm  No. 

Period  during  which 
measurements 
were  taken 

Number 
of 
people 

Volume 
per  person 
per  day 

1  

6-5-25 

gals. 

2  

9-4-25 
6-11-25 

3 

47.5 

3  

7-10-25 
5-4-26 

3 

17.0 

4.. 

8-3-26 
3-1-26 

4 

21.1 

5  

3-1-27 
6-15-25 

4 

30.4 

6..      .            ... 

12-21-25 
8-1-25 

7 

15.0 

71  

6-4-26 
6-19-25 

8 

10.0 

81  

6-19-26 
6_19_25 

5 

38.0 

91  

6-19-26 
6-19-25 

5 

45.8 

4-3-26 

4 

29.0 

JFarms  7,  8,  and  9  were  supplied  with  University  water  pressure  and  the  tenants 
were  not  charged  for  water  used. 

4.  The  quantity  of  sewage  flow  from  farm  homes  also  varies 
widely  with  the  habits  developed  in  the  use  of  water. 

5.  In  designing  septic  tanks  for  farm  homes  the  following  al- 
lowance for  average  sewage  flow  from  homes  of  different  sizes  is 
suggested : 

7  people,  25  gallons  per  capita  per  day 

9  people,  23  gallons  per  capita  per  day 
12  people,  20  gallons  per  capita  per  day 
15  people,  18  gallons  per  capita  per  day 


II— STUDY  OF  SINGLE-  AND  MULTIPLE-CHAMBER  SEPTIC 

TANKS:   EFFECT  OF  RETENTION  PERIOD 

ON  EFFLUENT 

Description  of  Experimental  Tanks 

In  order  to  compare  different  septic  tanks  while  treating  the  same 
kind  and  amount  of  sewage,  an  experimental  plant  was  constructed. 
Three  tanks  consisting  respectively  of  three  chambers,  two  chambers, 


304 


BULLETIN  No.  304 


[April, 


and  one  chamber  (A,  B,  and  C,  Fig.  4)  were  built  side  by  side,  and 
dosing  apparatus  (Fig.  5)  was  provided  so  that  each  tank  received 
the  same  dose  of  sewage. 

City  sewage  was  used  in  these  tanks  because  it  could  be  supplied 
in  equal  amounts  to  each,  thus  affording  a  comparison  of  different 
tanks  while  operating  under  the  same  conditions.  Each  of  the  three 
tanks  was  dosed  at  the  same  time  by  dividing  the  flow  from  a  central 
dosing  tank  (Fig.  5).  The  dosing  tank  was  supplied  from  the  Cham- 


A 


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D 


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1 

END    ELEVATfON 

FIG.  4. — PLAN  AND  END  ELEVATION  OF  EXPERIMENTAL 

SEPTIC  TANKS  USED  IN  STUDY  II 

Tanks  A,  B,  and  C  were  dosed  with  -city  sewage  and 
Tanks  D  and  E  were  connected  with  farm  homes. 

paign  city  sewer  with  ordinary  domestic  sewage  practically  free  from 
industrial  wastes.  The  city  water  consumption  was  about  80  gallons 
per  capita  per  day.1 

Each  tank  was  12  feet  long  and  3  feet  wide,  with  the  sewage 
standing  4  feet  deep  in  each  chamber. 

In  view  of  the  fact  that  the  three  tanks  described  above  were 


Tor  further  description  of  the  Champaign  sewage,  see  Bulletin  18,  Illinois 
State  Water  Survey,  pages  19  and  48. 


1928} 


EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS 


305 


dosed  with  city  sewage,  which  differs  from  farm  sewage,  additional 
septic  tanks  were  constructed  on  the  University  farm  and  connected 
to  farm  homes  in  order  to  secure  data  on  farm  sewage  simultaneously 
with  that  on  city  sewage.  A  two-chamber  tank  and  a  three-chamber 
tank,  identical  with  those  which  were  dosed  with  city  sewage,  were 
connected  to  farm  houses.  Plan  views  of  these  tanks  are  shown  at  D 
and  E,  Fig.  4,  and  the  details  common  to  all  the  tanks  in  Fig.  6. 


t "Orifice 


Qulck-opening 


Discharge  from  sewage  pump  — \ 


Dosing  Tank 


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FIG.  5. — PLAN  AND  ELEVATION  OF  THE  DOSING  APPARATUS 
This  dosing  tank  was  used  to  divide  the  flow  of  sewage  into  the  one-, 
two-,  and  three-chamber  tanks  (A,  B,  and  C)  shown  in  Fig.  4. 


The  tanks  treating  city  sewage  and  those  at  the  farm  homes  were 
all  put  in  operation  about  the  same  time.  Since  only  two  septic  tanks 
(two-  and  three-chamber)  were  connected  at  the  farm  homes  during 
this  test,  no  data  were  obtained  that  could  be  compared  with  those 
from  the  single-chamber  tank  which  treated  city  sewage. 

Dosing  Tanks  with  City  Sewage. — The  three  tanks  in  which  city 
sewage  was  used  were  dosed  by  an  attendant,  and  a  liquid  level  re- 


306 


BULLETIN  No.  304 


[April, 


corder  was  installed  in  the  dosing  tank  as  a  check.  Farm  conditions 
were  imitated  as  nearly  as  possible  by  dosing  the  tank  at  different 
times  of  the  day.  At  each  dosing  period  all  tanks  were  given  the  same 
kind  and  amount  of  sewage,  three  orifices  of  the  same  size  being 
located  in  the  bottom  of  the  dosing  tank  in  order  to  divide  the  sewage. 
Each  orifice  was  connected  to  a  septic  tank,  and  a  quick-opening  valve 
was  placed  in  the  sewer  line  to  each  tank  (Fig.  5) . 


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LONG  SECTION 

FIG.  6. — GENERAL  DESIGN  OF  EXPERIMENTAL  TANKS 

All  the  tanks  used  in  Studies  II  and  III  were  built  after  the  above  design, 
tho  there  were  certain  differences  in  detail.  In  Tank  C  the  partition  and  baffle 
at  Q  were  omitted;  in  Tanks  A  and  D  partitions  and  baffles  were  placed  at 
both  P  and  Q.  Tanks  F,  H,  I,  and  K  had  differently  shaped  cross-sections,  as 
shown  in  Fig.  17. 

The  tanks  were  dosed  by  first  pumping  a  definite  depth  of  sewage 
into  the  dosing  tank,  leaving  the  valves  closed  and  then  opening  all 
valves  together.  Each  tank  received  the  following  amounts  of  sewage 
daily:  8  a.m.,  55.5  gallons;  12  noon,  110.5  gallons;  5  p.m.,  110.5  gal- 
lons; 10  p.m.,  83  gallons;  or  a  total  charge  to  each  tank  in  24  hours  of 
359.5  gallons,  an  equivalent  of  48  cubic  feet  of  sewage.  This  was 
equal  to  the  volume  of  the  first  chamber  (Al)  of  the  three-chamber 
tank.  This  chamber,  therefore,  had  a  retention  period  of  24  hours  of 
sewage  flow;  the  first  chamber  (Bl)  of  the  two-chamber  tank  had  a 


1928}  EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS  307 

retention  period  of  48  hours;  and  the  single-chamber  tank  (C)  had  a 
retention  period  of  72  hours.  In  each  case  the  total  retention  period  of 
the  entire  tank  was  72  hours. 

The  maximum  tank  velocity  was  produced  by  the  dosings  at  noon 
and  at  5  p.m.,  when  110.5  gallons  of  sewage  was  admitted  to  each  tank 
in  4%  minutes.  The  rate  of  flow  to  the  septic  tanks  was  greatest  at 
the  beginning  of  these  dosings  and  gradually  decreased  as  the  head  on 
the  orifices  was  reduced.  The  maximum  rate  of  flow  at  the  beginning 
of  the  110.5-gallon  dosing  caused  a  tank  velocity  of  approximately  .3 
foot  a  minute. 

Flow  of  Sewage  to  Farm  Tanks. — The  two  tanks  connected  to  the 
farm  homes  received  sewage  which  varied  in  quality  and  amount  of 
flow.  The  water  consumption  for  each  home  was  metered,  and  the 
amount  used  was  taken  as  an  index  of  the  sewage  flow. 

The  three-chamber  tank  (D)  received  sewage  from  a  home  of 
five  people,  and  the  average  sewage  flow  was  140  gallons  a  day,  or  28 
gallons  per  capita  per  day. 

The  two-chamber  tank  (E),  which  received  sewagje  from  a  home 
of  five  people,  had  an  average  sewage  flow  of  650  gallons  a  day,  or  130 
gallons  per  capita  per  day.  The  large  flow  to  this  tank  was  due  to  a 
leak  in  the  toilet,  which  was  discovered  and  stopped  March  20,  1924, 
after  the  first  study  on  the  tank  had  been  completed.  The  water  con- 
sumption then  dropped  from  130  to  40  gallons  per  capita  per  day.  The 
fact  that  during  this  test  the  sewage  treated  by  this  two-chamber  tank 
was  diluted  with  a  large  quantity  of  water  should  be  taken  into  con- 
sideration in  making  comparisons  with  the  other  tanks  in  Tables  3, 
15,  and  16. 

Collection  of  Samples. — Samples  were  collected  for  chemical 
analysis  from  each  chamber  of  each  tank  every  six  days.  The  samples 
did  not  actually  flow  from  the  chambers,  but  were  collected  at  the 
outlets  by  means  of  a  sampling  device.  The  sampling  points  are  indi- 
cated at  al,  b2,  etc.,  in  Fig.  4.  Two  liters  of  sewage  were  collected  for 
each  sample.  The  sampling  device  consisted  of  a  galvanized  iron 
cylinder  with  an  inlet  at  the  bottom  for  admitting  sewage  (Fig.  7). 
The  device  drew  the  sample  from  a  depth  approximately  17  inches 
below  the  sewage  level  in  the  tank.  In  this  way  samples  were  obtained 
free  from  scum. 

The  sample  was  taken  by  forcing  the  sampler  below  the  level  of 
the  sewage  in  the  tank,  pulling  on  trigger  B  so  as  to  open  valve  A,  and 
allowing  sewage  to  flow  into  the  cylinder  and  displace  air  thru  the 
tube  C.  The  charge  of  sewage  was  then  transferred  from  the  sampler 
to  a  2.5-liter  bottle. 

In  collecting  samples  from  the  first  chambers  of  the  tanks,  the 
sewage  sampler  was  inserted  into  the  vertical  tile  tee,  as  shown  in 
plan  at  A  in  Fig.  6;  in  collecting  samples  at  the  outlet  of  the  last 


308 


BULLETIN  No.  304 


[April, 


chamber  the  sampler  was  inserted  between  the  baffle  and  the  end  of 
the  tank,  as  shown  in  plan  at  B.1  Usually  there  was  no  scum  accumu- 
lation at  the  sampling  points;  if  scum  was  present,  it  was  avoided 
while  inserting  the  inlet  of  the  sampling  device. 

No  definite  hour  of  the  day  was  set  for  collecting  the  samples 
from  the  tanks  connected  to  farm  homes.  Sometimes  effluent  would 
be  passing  from  the  tank  while  the  sample  was  being  collected,  and  at 
other  times  there  would  be  none,  depending  on  the  flow  of  sewage 


•—  galvanized  iron 


capacity  2  liters 


•  helical  Spring 


Section  art  DD 

FIG.   7. — SEWAGE   SAMPLING  DEVICE 


from  the  house  to  the  tank.  Likewise,  no  definite  hour  of  the  day  was 
set  for  collecting  the  samples  from  the  tank  treating  city  sewage. 
Samples  were  not  collected,  however,  while  the  tanks  were  being  dosed, 
because  of  the  difficulty  of  getting  all  samples  at  the  same  stage  of 
the  dosing  operation. 

Altho  the  above  method  of  collecting  samples  was  not  ideal,  the 
samples  were  representative  of  effluent  from  tanks  operating  under 
farm  conditions.  Under  ordinary  farm  service  a  septic  tank  operates 

*In  future  experimentation  the  provision  of  a  slight  fall  between  chambers 
and  at  the  tank  outlet  will  permit  the  use  of  a  receptacle  to  collect  samples  of 
effluent  as  they  flow  from  the  different  chambers. 


1928}  EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS  309 

intermittently — sometimes  there  is  no  discharge,  sometimes  a  very 
slow  discharge  owing  to  the  contents  of  the  tank  being  displaced  by 
flow  from  a  lavatory  or  a  kitchen  sink,  and  sometimes  a  more  rapid 
discharge  because  of  bathtub  or  toilet  drainage.  The  samples  collected 
from  the  city-sewage  tanks  soon  after  dosing  were  representative  of 
the  effluent  from  a  farm  tank  which  results  from  bathtub  or  toilet  dis- 
charge, and  those  collected  a  considerable  time  after  dosing  were  rep- 
resentative of  effluent  from  a  farm  tank  caused  by  sink  drainage. 

The  samples  from  the  three  tanks  in  which  city  sewage  was 
treated  were  taken  every  six  days.  A  separate  sample  was  collected 
from  each  one  of  the  six  sampling  points.  One  sample  was  collected 
immediately  after  the  other,  all  six  being  taken  in  10  to  30  minutes. 
As  the  sampling  dates  were  six  days  apart,  each  day  of  the  week  was 
represented  by  the  samples.  Likewise,  the  two  tanks  at  the  farm 
houses  were  sampled  at  each  point  every  six  days.  In  order  to  dis- 
tribute the  work  of  analysis,  the  two  sets  of  samples,  representing  city 
sewage  and  farm  sewage,  were  cpllected  three  days  apart. 

Analytical  Methods. — The  following  determinations  were  made  on 
the  samples  in  the  laboratories  of  the  State  Water  Survey:  chlorin  in 
chlorids,  alkalinity,  ammonia  nitrogen,  nitrate  nitrogen,  nitrite  nitro- 
gen, oxygen  consumed,  turbidity,  residue  on  evaporation,  settleable 
solids  (Imhoff  cone). 

The  analytical  methods  were  those  prescribed  by  the  American 
Public  Health  Association  and  the  American  Water  Works  Associa- 
tion, "Standard  Methods  for  the  Examination  of  Water  and  Sewage, 
1925."  Results  are  reported  in  parts  per  million,  with  the  exception  of 
settleable  solids,  which  are  reported  in  cubic  centimeters  per  liter. 

Of  the  determinations  made  on  the  samples,  those  used  in  judging 
the  relative  performance  of  the  different  tanks  were  settleable  solids, 
total  residue  on  evaporation,  turbidity,  and  oxygen  consumed  from 
permanganate.  The  first  three  determinations  give  a  measure  of  the 
suspended  and  dissolved  solids  which  the  tank  does  not  remove.  The 
permanganate  test  measures  the  oxidizable  material  in  the  effluent,  and 
therefore  gives  a  further  measure  of  its  quality. 

Measurement  of  Scum  and  Sludge. — At  the  end  of  the  test  the 
total  accumulation  of  scum  and  sludge  was  measured  in  each  chamber 
as  follows: 

The  thickness  of  the  scum  was  determined  by  the  device  shown  in  Fig.  8, 
which  consists  of  a  metal  plate  (D)  mounted  on  the  end  of  a  %-inch  pipe  so  that 
the  plate  can  be  moved  from  a  vertical  to  a  horizontal  position  (or  vice  versa) 
by  means  of  the  wire  control  (E).  The  different  positions  of  the  plate  are  shown 
by  the  three  views  in  Fig.  8.  The  plate  (in  vertical  position)  is  forced  down 
thru  the  scum  and  then  turned  to  the  horizontal  position.  The  plate  is  then 
raised  until  it  comes  into  contact  with  the  undersurface  of  the  scum,  and  the 
measuring  rod  is  placed  in  a  vertical  position  with  one  end  in  contact  with  the 
upper  surface  of  the  scum.  A  reading  of  the  index  (F)  on  the  measuring  rod 


310 


BULLETIN  No.  304 


[April, 


(G)  then  gives  directly  the  thickness  of  the  scum.    The  measuring  rod  is  gradu- 
ated to  feet  and  hundredths  of  a  foot  reading  downward  from  the  top. 

The  surface  line  of  the  sludge  at  the  bottom  of  the  tank  was  located  by 
the  use  of  a  small  bottle  mounted  on  the  end  of  a  graduated  rod  as  shown  in 
Fig.  9.  The  rod  (A)  is  graduated  to  feet  and  hundredths  of  a  foot  from  the  top 
down.  The  valve  at  the  mouth  of  the  bottle  is  opened  by  the  handle  (B).  The 
length  of  the  steel  rod  (C)  is  such  that  when  one  end  of  it  rests  on  the  bottom 
of  the  tank,  the  other  end  indicates  directly  on  the  graduated  rod  (A)  the  dis- 
tance between  the  bottom  of  the  tank  and  the  mouth  of  the  bottle.  The  sludge 


FIG.  8. — DEVICE  FOR  MEASURING 
THE  THICKNESS  OF  SCUM 

line  is  determined  with  this  device  by  first  taking  a  sample  from  the  clear  liquid 
above  the  sludge  line  and  then  taking  samples  a  little  deeper  each  time  until 
the  bottle  is  filled  with  dark  liquid.  The  reading  of  the  upper  end  of  the 
steel  rod  (C)  on  the  graduated  rod  (A)  at  this  last  depth  gives  directly  the 
depth  of  sludge  in  the  bottom  of  the  tank. 

In  making  the  scum  and  sludge  measurements  on  the  chambers 
which  were  more  than  4  feet  long,  two  or  three  complete  readings  were 
taken  and  the  average  used  in  computing  the  total  scum  and  sludge 
for  these  chambers. 

Temperature  Records. — The  temperature  of  the  sewage  in  each 
chamber  was  taken  at  the  time  each  sample  was  taken. 

Duration  and  Conditions  of  Tests. — The  dosing  of  the  three  tanks 
in  which  city  sewage  was  treated  (Tanks  A,  B,  C)  started  November 


1928] 


EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS 


311 


22,  1922.  There  was  some  leakage  from  the  tanks  for  a  short  time  un- 
til the  pores  in  the  concrete  became  filled ;  sewage  started  to  flow  from 
the  tank  outlets  December  11,  1922.  Effluent  samples  were  collected 
from  December  12,  1922,  to  December  18,  1923. 

The  three-chamber  tank  (D),  which  treated  farm  sewage,  was 
connected  November  16,  1922.  The  tank  leaked  for  a  considerable 
time  after  being  put  into  operation ;  no  effluent  flowed  over  the  outlet 
until  February  12,  and  the  sewage  level  was  below  the  outlet  at  times 


LEATHER  WASHEI 


BOTTLE.  WIWD  TO 

^RADUATtD  ROD -7  < 


ANGLE.  BPACL 
FIG.  9. — DEVICE  FOR  MEASURING  THE  DEPTH  OF  SLUDGE 


until  March  8,  1923.  Samples  were  collected  December  15, 1922,  to  De- 
cember 19,  1923.  This  tank  treated  sewage  from  five  people,  and  the 
sewer  connections  consisted  of  a  toilet,  a  kitchen  sink,  and  a  bathtub. 

The  two-chamber  tank  (E)  which  treated  farm  sewage  was  con- 
nected November  18,  1922,  and  sewage  flowed  from  the  outlet  Novem- 
ber 27.  Effluent  samples  were  collected  from  December  15,  1922,  to 
December  19,  1923.  This  tank  treated  sewage  from  five  people,  the 
connections  consisting  of  a  toilet,  a  kitchen  sink,  a  bathtub,  and  a 
laundry  drain. 

During  this  investigation  conditions  were  abnormal  in  Tanks  D 
and  E,  in  which  farm  sewage  was  treated,  the  influent  to  Tank  E 


312 


BULLETIN  No.  304 


\April, 


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314 


BULLETIN  Xo.  304 


[April, 


TABLE  4. — AVERAGE  ANALYSES  OF  54  SAMPLES  OF  EFFLUENT  FROM  SEPTIC  TANK 
CHAMBERS  OF  DIFFERENT  RETENTION  PERIODS  (Crrr  SEWAGE)  l 

(54  samples,  collected  December  12,  1922,  to  December  18,  1923) 


Chamber  No  

Al 
24-hour  capacity 

Bl 

48-hour  capacity 

C 
72-hour  capacity 

(4'x3'x4') 

(8'x3'x4') 

(12'x3'x4') 

Oxygen  consumed  

47.3 

48.4 

45  3 

Turbidity  

251 

190 

177 

Residue  on  evaporation.  .  .  . 
Settleable  solids  

1265 
3.40 

1095 
1.54 

1136 
1.97 

Total  scum  and  sludge, 
cu.  ft  

31.8 

39.6 

43.6 

Comparison  of  effluents  from  sampling  points  a/,  bl,  and  c  in  Fig.  4.  In  effect, 
each  chamber  functions  as  a  single-chamber  tank  or  as  the  first  chamber  of  a  mul- 
tiple-chamber tank. 


TABLE  5. — AVERAGE  ANALYSES  OF  54  SAMPLES  OF  EFFLUENT  FROM  SINGLE-  AND 
MULTIPLE-CHAMBER   SEPTIC   TANKS   OF   72-HouR  TOTAL   CAPACITY   (CrrY 

SEWAGE)1 

(Collected  December  12,  1922,  to  December  18,  1923) 


Sampling  point  

aS 
(3-chamber  tank) 

b2 
(2-chamber  tank) 

c 
(1  -chamber  tank) 

Oxvgen  consumed  : 

42.8 

46.2 

45.3 

Turbidity  

140 

157 

177 

Residue  on  evaporation.  .  .  . 
Settleable  solids  

1000 
1.18 

1000 
1.00 

1136 
1.97 

Total  scum  and  sludge,2 
cu.  ft  

55.2 

45.0 

43.6 

Comparison  of  effluents  from  sampling  points  a3, 
2Total  accumulation  in  all  chambers. 


?,  and  c  in  Fig.  4. 


TABLE  6. — AVERAGE  ANALYSES  OF  54  SAMPLES  OF  EFFLUENT  FROM  SINGLE-  AND 
MULTIPLE-CHAMBER  SEPTIC   TANKS   OF  48-HouR  TOTAL   CAPACITY    (Crrr 

SEWAGE)  1 

(Collected  December  12,  1922,  to  December  18,  1923) 


Sampling  point  

a2 
(2-chamber  tank) 

bl 
(1  -chamber  tank) 

Oxygen  consumed  

48.0 

48.4 

Turbidity  

192 

190 

Residue  on  evaporation  

1180 

1095 

Settleable  solids  

2.64 

1.54 

Total  scum  and  sludge,  cu.  ft.2  

46.8 

39.6 

Comparison  of  effluents  from  sampling  points  a2  and  bl  in  Fig.  4. 
"Total  accumulation  in  all  chambers. 


1928]  EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS  315 

being  diluted  by  a  leaky  toilet  and  Tank  D  leaking  until  the  pores  in 
the  concrete  became  filled.  However,  sufficient  data  were  obtained  to 
justify  the  drawing  of  conclusions. 

Results  Obtained 

The  data  given  in  Tables  2  and  3,  showing  briefly  the  conditions 
during  the  study  and  the  average  chemical  analyses  of  the  effluent 
over  the  entire  test,  make  it  possible  to  study  the  effect  of  variation 
in  retention  period  on  the  efficiency  of  operation,  and  the  comparative 
efficiency  of  single-chamber  and  multiple-chamber  tanks.  Information 
on  the  functioning  of  the  tanks  at  two  different  stages  of  their  oper- 
ation is  given  in  Tables  15  and  16  in  the  Appendix. 

In  making  comparisons  of  data  from  Tables  3,  15,  and  16,  it 
should  be  remembered  that  the  tanks  treating  city  sewage  received 
the  same  kind  and  amount  of  sewage  thruout  the  period,  while  the 
tanks  connected  to  farm  houses  received  sewage  which  varied  con- 
siderably in  quality  and  in  amount  of  flow.  Direct  comparisons  there- 
fore can  hardly  be  made  of  the  results  obtained  from  the  city  sewage 
and  those  from  the  farm  sewage. 

Tables  4,  5,  and  6  are  compiled  from  Table  3,  the  results  secured 
with  chambers  of  different  capacity  and  retention  period  being  given 
in  Table  4;  those  secured  with  one-,  two-,  and  three-chamber  tanks 
of  72-hour  total  retention  period  in  Table  5;  and  those  secured  with 
one-  and  two-chamber  tanks  of  48-hour  retention  period,  in  Table  6. 

Data  on  sludge  accumulation  for  the  different  chambers  are  shown 
in  Tables  7  and  8.  Figs.  10  to  16  show  curves  with  tank  temperature, 
turbidity,  residue  on  evaporation,  and  settleable  solids  plotted  against 
time  for  the  following  chambers:  Al,  Bl,  B2,  C,  Dl,  El,  and  E2. 

Discussion  and  Conclusions 

A  study  of  the  data  collected  in  this  second  investigation  revealed 
the  following  facts  and  leads  to  the  conclusions  and  recommendations 
indicated. 

1.  The  chamber  with  a  48-hour  capacity  (Bl)  showed  a  marked 
improvement  over  the  chamber  having  a  24-hour  capacity,  (Al)  as 
indicated  by  lower  turbidity,  less  residue  on  evaporation,  and  less 
settleable  solids  (Table  4) .  As  between  Chamber  C,  having  a  72-hour 
capacity,  and  Chamber  Bl,  having  a  48-hour  capacity,  the  72-hour 
tank  had  the  advantage  of  lower  oxygen  consumption  and  lower  turbid- 
ity. On  the  other  hand,  the  48-hour  tank  showed  less  residue  on 
evaporation,  less  settleable  solids,  and  a  smaller  scum  and  sludge  ac- 
cumulation. These  last  three  points,  together  with  the  smaller  cost  of 
the  48-hour  tank,  would  give  it  the  advantage  for  the  first  year's 
operation,  but  would  make  no  provision  for  sludge  storage  over  a 
period  of  years.  In  the  design  of  a  single-chamber  tank  or  of  the  first 
chamber  of  a  multiple-chamber  tank,  an  allowance  might  well  be  made 


316 


BULLETIN  No.  304 


[April, 


for  a  48-hour  effective  retention  period  with  50  percent  additional 
capacity  for  sludge  storage,  or  a  total  retention  period  of  72  hours. 
This  would  insure  efficient  operation  for  a  longer  period  without  the 
necessity  of  cleaning  the  tank. 

2.  Of  the  tanks  with  a  total  retention  period  of  72  hours  and  with 
the  same  dosing  of  city  sewage,  the  two-chamber  tank  (B)  gave  the 
best  results  (Table  5) .  The  advantage  of  the  two-chamber  tank  over 
the  three-chamber  tank  was  evidently  due  to  the  fact  that  the  retention 

TABLE  7. — SCUM  AND  SLUDGE  ACCUMULATION  IN  CHAMBERS  OF  TANKS  TREATING 

CITY  SEWAGE  DURING  STUDY  OF  SINGLE-  AND  MULTIPLE-CHAMBER 

SEPTIC  TANKS1 


Chamber  No  

Al 

A2 

A3 

Bl 

B2 

C 

(4'x3') 

(4'x3') 

(4'x3') 

(8'x3') 

(4'x3') 

(12'x3') 

Depth  of  scum  

ft. 
1.90 

ft. 
.45 

ft- 
.10 

ft. 
1.00 

ft. 
.05 

ft. 

.63 

.75 

.80 

.60 

.65 

.40 

.58 

Total  depth  of  scum  and  sludge.  .  .  . 
Volume  of  scum  and  sludge  

2.65 

cu.  ft. 
31.8 

1.25 

cu.  ft. 
15.0 

.70 

cu.  ft. 
8.4 

1.65 

cu.  ft. 
39.6 

.45 
cu.  ft. 
5.4 

1.21 
cu.  ft. 
43.5 

"The  tanks  were  put  in  operation  November  22,   1922,  and  the  above  measurements  made 
December  17,  1923. 


TABLE  8. — SCUM  AND  SLUDGE  ACCUMULATION  IN  CHAMBERS  OF  TANKS  TREATING 

FARM  SEWAGE  DURING  STUDY  OF  SINGLE-  AND  MULTIPLE-CHAMBER 

SEPTIC  TANKS1 


Chamber  No  

Dl 

D2 

D3 

El 

E2 

(4'x3') 

(4'x3') 

(4'x3') 

(8'x3') 

(4'x3') 

Depth  of  scum  

ft. 
.63 

ft. 
.01 

ft. 
0.00 

ft. 
.21 

ft. 
.02 

.40 

.50 

1 

.60 

.40 

1.03 

.51 

.81 

.42 

Volume  of  scum  and  sludge  

cu.  ft. 
12.4 

CU.  ft. 

6.1 

cu,.  ft. 
19.4 

cu.  ft. 

5.05 

"The  tanks  were  put  into  operation  November  16  and  18,  1922,  and  the  above  measurements 
were  made  December  17,  1923. 

'The  sludge  in  chamber  D3  was  less  than  .4  foot  deep  and  could  not  be  measured  conveniently. 

period  and  distribution  of  scum  and  sludge  in  each  chamber  of  the 
two-chamber  tank  was  most  favorable  for  settlement  and  digestion  of 
solids  by  bacterial  action.  The  advantage  of  the  two-  and  three- 
chamber  tanks  over  the  single-chamber  was  no  doubt  due  to  additional 
baffling ;  furthermore,  most  of  the  sludge  in  the  two-  and  three-cham- 
ber tanks  was  stored  in  the  first  chambers,  and  considerable  gassing 
and  disturbance  was  thereby  eliminated  near  the  outlet  of  these  tanks 
(Table  7). 

3.  With  tanks  of  48-hour  total  retention  period,  the  single-cham- 
ber tank  gave  better  results  than  the  two-chamber  tank,  as  evidenced 
by  study  of  effluents  from  a2  and  bl  (Table  6  and  Fig.  4).  The 
unfavorable  results  with  the  small  two-chamber  tank  were  probably 


1928} 


EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS 


317 


due  to  the  fact  that  after  operating  for  a  time,  a  large  part  of  the  first 
chamber  was  occupied  by  scum  and  sludge,  thus  reducing  the  effective 
capacity  and  retention  period  below  that  required  for  proper  settle- 
ment and  digestion  of  solids  (Table  7) . 

With  tanks  of  a  72-hour  total  capacity  there  was  an  advantage 
in  using  two  chambers,  but  with  tanks  having  a  48-hour  total  capacity 


Jan  February        March 


April 


August     Jepfember      October       NOK 


FIG.  10. — VARIATIONS  IN  EFFLUENT  FROM  CHAMBER  Al,  FIRST  CHAMBER  OF 
TANK  A  (FiG.  4) :  CITY  SEWAGE 

two  chambers  were  no  advantage.  In  other  words,  a  two-chamber 
tank  is  desirable  if  the  tank  is  of  sufficient  size  to  provide  a  48-hour 
capacity,  or  more,  in  the  first  chamber. 

4.  Each  additional  chamber  produced  considerable  improvement 
in  the  quality  of  the  effluent  over  that  produced  by  the  preceding 
chamber.     Similar  results  were  obtained  with  both  city  and  farm 
sewage  (Table  3) . 

5.  Curves  showing  tank  temperature,  turbidity,  residue  on  evapor- 
ation, and  settleable  solids  plotted  against  time  for  chambers  Al, 
Bl,  C,  Dl,  and  El  (Figs.  10,  11,  13,  14,  15)  indicate  that  the  effluent 
was  relatively  high  in  solids  during  the  summer  months  while  the 
tank  temperature  was  high. 

6.  Curves  showing  tank  temperature  and  settleable  solids  plotted 
against  time  for  chambers  Al,  Bl,  B2,  and  C  (Figs.  10  to  13)  indi- 
cate higher  settleable  solids  after  several  months'  operation  even  for 
periods  with  approximately  the  same  tank  temperature.  This  increase 
was  probably  the  result  of  scum  and  sludge  accumulation. 


318 


BULLETIN  No.  304 


[April, 


7.  Curves  with  turbidity,  residue  on  evaporation,  and  settleable 
solids  plotted  against  time  show  considerable  variation  in  the  quality 
of  the  effluent,  especially  for  settleable  solids  (Figs.  10,  11,  13,  14,  15). 


Jan  February     March          Apr//  May  Ja 


July  /Icjgusf    September   Ocfobtr 


FIG.  11. — VARIATIONS  IN  EFFLUENT  FROM  CHAMBER  Bl,  FIRST  CHAMBER  OF 
TANK  B:  CITY  SEWAGE 


Jan   February       Marc/>  April  May  June  July          August    September         October      /Vo, 


FIG.  12. — VARIATIONS  IN  EFFLUENT  FROM  CHAMBER  B2,  SECOND  CHAMBER  OF 
TANK  B:  CITY  SEWAGE 


1928} 


EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS 


319 


This  variation  is  undoubtedly  due  to  the  fact  that  as  sludge  accu- 
mulates, gassing  takes  place  intermittently  and  causes  the  discharge 


£ 

SMI 


February      March  April  May 


July  August     September    October     No* 


FIG.  13.  —  VARIATIONS  IN  EFFLUENT  FROM  CHAMBER  C:    CITY  SEWAGE 


rebruary       March  Apr//  May 


July  August    Japtember     Ociobt 


FIG.  14. — VARIATIONS  IN  EFFLUENT  FROM  CHAMBER  Dl,  FIRST  CHAMBER  OF 
TANK  D:  FARM  SEWAGE 


320 


BULLETIN  No.  304 


[April, 


-g" 


a 

400 
-6 
^°° 

xj 
C«W 

v5 


1200 

§ 

")  «00 

5 

«w 

I/J 

iS/0 


C/fAMBCff 


Tank   Temp 


v 


February       March  April  May  June  July 

FIG.  15.  —  VARIATIONS  IN  EFFLUENT  FROM  CHAMBER 
TANK  E:  FARM  SEWAGE 


August      September      October 

El,  FIRST  CHAMBER  OF 


EFFLUENI 


jt* 


'd/ty 


l\        A 


V 


yv\ 


•8* 


Settle  a bl, 


Solids 


February        March          April  May  June  July  August       September       October 

FIG.  16. — VARIATIONS  IN  EFFLUENT  FROM  CHAMBER  E2,  SECOND  CHAMBER  OF 
TANK  E:  FARM  SEWAGE 


1928}  EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS  321 

of  considerable  amounts  of  flock  at  one  time,  thus  producing  a  marked 
variation  in  the  quality  of  the  effluent. 


Ill— STUDY   OF   TWO-CHAMBER  SEPTIC   TANKS   HAVING 
DIFFERENTLY  SHAPED  CROSS-SECTIONS 

The  purpose  of  this  third  investigation  was  to  secure  data  on  the 
effect  of  differences  in  shape  of  cross-section  on  the  efficiency  of  septic 
tanks  and  to  secure  additional  data  on  the  performance  and  behavior 
of  two-chamber  septic  tanks.  Comparisons  were  made  between  two- 
chamber  tanks  of  the  same  capacity  that  differed  in  shape  of  cross- 
section. 

Description  of  Experimental  Tanks 

Three  two-chamber  septic  tanks  of  different  cross-sections  were 
compared  while  treating  city  sewage,  being  dosed  with  the  same  ap- 
paratus as  that  used  in  Investigation  No.  2.  The  two-chamber  tank 
which  was  used  to  treat  city  sewage  in  the  previous  study  was  cleaned 
out  and  used  again  in  this  test.  The  other  two  tanks  were  rebuilt,  so 
that  all  three  were  compared  as  two-chamber  tanks  having  the  same 
capacity  but  differing  in  shape  of  cross-section.  Plan  views  of  the 
tanks  are  shown  at  F,  G,  and  H  in  Fig.  17,  and  general  details  in  Fig. 
6.  A  bird's-eye  view  is  given  in  Fig.  18. 

The  first  chamber  of  each  tank  was  8  feet  long  and  the  second  4 
feet  long.  The  tanks  were  of  the  following  cross-section:  narrow  tank, 
2.4  feet  wide,  5-foot  depth  of  sewage;  medium  tank,  3  feet  wide,  4- 
foot  depth  of  sewage;  wide  tank,  4  feet  wide,  3-foot  depth  of  sewage. 
The  cross-sectional  area  and  capacity  of  each  tank  was,  of  course,  the 
same. 

Three  tanks  treating  farm  sewage,  identical  with  those  treating 
city  sewage,  were  included  in  the  study.  The  two-chamber  tank  used 
in  the  first  investigation  to  treat  sewage  from  one  of  the  farm  homes 
was  cleaned  out  and  operated  again;  the  other  tank  on  the  University 
farm  was  rebuilt;  and  a  third  one  was  constructed  and  connected  to  a 
house  on  the  University  farm.  Plan  views  of  the  tanks  are  shown  at 
I,  J,  and  K  in  Fig.  17,  and  general  details  in  Fig.  6.  The  three  tanks 
treating  city  sewage  and  the  three  treating  farm  sewage  were  put  into 
operation  at  about  the  same  time. 

Dosing  Tanks  with  City  Sewage. — Dosing  with  city  sewage  was 
carried  on  in  the  same  manner  as  in  the  previous  investigation.  A 
daily  charge  of  359.5  gallons  was  made  to  each  tank.  This  was 
equivalent  to  a  total  retention  period  in  each  tank  of  72  hours. 

Flow  of  Sewage  to  Farm  Tanks. — The  three  tanks  installed  at 
farm  homes  received  sewage  which  varied  in  quality  and  in  amount 
of  flow.  The  tank  of  narrow  cross-section  (I)  received  sewage  from  a 
home  of  four  people,  in  which  the  average  flow  was  95  gallons  a  day 


322 


BULLETIN  No.  304 


[April, 


(23.7  gallons  per  capita).  The  medium-width  tank  (J)  received  sewage 
from  a  home  of  four  to  six  people  (average  of  five  people),  in  which 
the  average  sewage  flow  was  219  gallons  a  day  (43.7  gallons  per 
capita).  The  wide  tank  (K)  received  sewage  from  a  home  of  five 
people,  and  the  average  flow  was  175  gallons  a  day  (35  gallons  per 
capita) . 


r 


r 


j 


K 


Q 

1 


Ft 

Gi 

m 

It 

Jl 

Ki 

ft 

hi 

ii 

Jl 

lei 

F2 

G2 

H2 

12 

Jz 

Kt 

ft 

S2 

hz 

12 

J2 

kz 

1          1       ,<~l              1            1               1 

Outlet  —  -^ 

PLAN 

1  24'—*  j-'tf-  —  |  4-0'  1  H--?.4^  +—$0~—\   H  4-0"  H 

? 

1 

~ 

S 

5 

» 

5? 

10 

n 

^ 

Y 

1 

i> 

t 

^ 

ELEVATION 

FIG.  17. — PLAN  AND  END  ELEVATION  OF  EXPERIMENTAL  SEPTIC  TANKS  USED 

IN  STUDY  OF  DIFFERENTLY  SHAPED  CROSS-SECTIONS 

Tanks  F,  G,  and  H  were  dosed  With  city  sewage  and  Tanks  I,  J,  and  K 
were  connected  with  farm  homes.  These  tanks  differ  from  those  used  in  Study  II 
(Fig.  4)  in  the  relative  shape  of  cross-section  and  in  the  number  of  chambers  per 
tank.  The  dimensions  indicate  inside  measurements  except  those  given  for  depth, 
which  indicate  the  depth  of  sewage  standing  in  the  tank. 


Collecting  and  Analyzing  Samples. — Samples  were  collected  in 
the  same  manner  as  in  the  study  of  single-  and  multiple-chamber 
tanks,  except  that  they  were  taken  from  each  sampling  point  every 
eight  days  instead  of  every  six.  The  sampling  dates  were  for  each 
day  of  the  week  in  rotation,  that  is,  Sunday,  Monday,  Tuesday,  etc. 
The  method  of  chemical  analysis  also  was  the  same. 


1928] 


EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS 


323 


Measurements  of  Scum  and  Sludge. — Scum  and  sludge  measure- 
ments were  made  in  the  same  manner  as  in  the  investigation  on  single- 
and  multiple-chamber  tanks  except  that  they  were  taken  at  intervals 
thruout  the  study  in  order  to  secure  data  on  the  rate  of  accumulation. 
The  measurements  were  continued  on  the  tanks  at  the  farm  homes 
for  14  months  after  the  analysis  of  effluent  was  discontinued,  in  order 
to  secure  data  over  a  period  of  several  years. 

Duration  and  Conditions  of  Test. — During  this  study  of  cross- 
sections  the  tanks  treating  both  city  and  farm  sewage  were  discharg- 


fet 


FIG.  18. — BIRD'S-EYE  VIEW  OF  EXPERIMENTAL  SEPTIC  TANKS  F,  G,  AND  H 

DOSED  WITH  CITY  SEWAGE 
I 

ing  from  the  outlets  shortly  after  being  put  into  operation.  The  dosing 
and  sampling  of  the  three  tanks  treating  city  sewage  was  started  early 
in  July,  1924,  and  was  continued  until  October  12,  1925,  except  for  a 
period  of  4%  months  (from  October  30,  1924,  to  the  middle  of  March, 
1925),  when  no  attendant  was  available  to  take  care  of  the  dosing. 
Scum  and  sludge  measurements  made  before  the  tanks  were  shut  down 
and  at  the  time  they  were  put  into  operation  again,  indicated  a  partial 
settlement  of  scum  but  little  change  in  the  aggregate  volume  of  scum 
and  sludge  while  the  tanks  were  idle  (Fig.  21). 

Effluent  samples  were  collected  from  the  tanks  treating  farm  sew- 
age from  July  8,  1924,  to  December  11,  1925,  except  from  October  to 
March  while  the  city  sewage  tanks  were  shut  down.  The  tank  of 
narrow  cross-section  (I),  treating  sewage  from  four  people  in  a  farm 
home  where  the  plumbing  fixtures  consisted  of  a  toilet,  a  kitchen  sink, 
a  bathtub,  a  lavatory,  and  a  laundry  drain,  was  connected  June  4, 
1924.  The  medium  tank  (J),  which  treated  sewage  from  four  to  six 
people  in  a  farm  home  where  the  fixtures  consisted  of  a  toilet,  a  kitchen 
sink,  a  bathtub,  and  a  laundry  drain,  was  connected  June  12,  1924. 
The  wide  tank  (K) ,  treating  sewage  from  five  people  in  a  farm  home 


324 


BULLETIN  No.  304 


[April, 


where  the  fixtures  consisted  of  a  toilet,  a  kitchen  sink,  and  a  bathtub, 
was  connected  June  9,  1924. 


I- 

*/«» 


\ 


TANK  G.CrnuatT  nto/nfaa, 


/ 

/ 


AND  3rc< 


NDC/WI& 


^  SeH><abk  Jofefe. 


ryavber  'ffj 


yi//y     August  Septvmlxir  Octolxr   —    /«tor     X/vv/         V. 


August  Jep/emter  Oct 


FIG.  19. — VARIATIONS  IN  EFFLUENT  FROM  CHAMBERS  Gl  AND  G2  OF 
TANK  G  (FiG.  17) :  CITY  SEWAGE 


KJ.  'CtTLUOT  rpoit  fwsr  AND^KOND  CH.WKKS 


Temperoh 


g/aw 

$000 


\/\ 


Saoo 
*L 


frqa.  On.  iterji 


Juf?       August  Jytftmtcr    Oc/oter     —    Mar.        jpri/         ifaj>        June        My         August  September  Ocf-oter     Nov.     OK. 

FIG.  20. — ^VARIATIONS  IN  EFFUENT  FROM  CHAMBERS  Jl  AND  J2 
OF  TANK  J:   FARM  SEWAGE 


EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS 


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326 


BULLETIN  No.  304 


[April, 


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1928] 


EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS 


327 


Results  Obtained 

A  basis  for  studying  the  effect  of  shape  of  cross-section  on  the 
operation  of  septic  tanks  may  be  found  in  the  analytical  data  sum- 
marized in  Table  10,  which  shows  average  chemical  analyses  of  effluent 

TABLE  11. — SCUM  AND  SLUDGE  ACCUMULATION  IN  CHAMBERS  OF  TANKS  TREATING 
CITY  SEWAGE  DURING  STUDY  OF  TANK  CROss-SEcriONS1 


Chamber  No.  

Fl 

F2 

Gl 

G2 

Hi 

H2 

(8'x2.4'x5') 

(4'x2.4'x5') 

(8'x3'x4') 

(4'x3'x4') 

(8'x4'x3') 

(4'x4'x3') 

Depth  of  scum  

ft. 
1.13 

ft. 
.05 

ft. 
.88 

ft. 
.05 

ft. 
.61 

ft. 
05 

Depth  of  sludge  

1.31 

.70 

1.18 

.55 

1.03 

.55 

Total  depth  of  scum  and  sludge 
Volume  of  scum  and  sludge.  .  .  . 

2.44 
cu.  ft. 
46.8 

.75 

cu.  ft. 
7.2 

2.06 
cu.  ft. 
49.5 

.60 

cu.ft. 
7.2 

1.64 

CM.  ft. 

52.5 

.60 
cu.  ft. 
9.6 

'The  tanks  were  put  in  operation  June  30,  1924,  and  the  above  measurements  were  made  Octo- 
ber 14,  1925.    Tanks  were  idle  October,  1924,  to  March,  1925. 

TABLE  12. — SCUM  AND  SLUDGE  ACCUMULATION  IN  CHAMBERS  OF  TANKS  TREATING 
FARM  SEWAGE  DURING  STUDY  OF  TANK  CROss-SEcnoN1 


Chamber  No  

11 

12 

J] 

J2 

Kl 

K2 

(8'x2.4'x5') 

(4'x2.4'x5') 

(8'x3'x4') 

(4'x3'x4') 

(8'x4'x3') 

(4'x4'x3') 

Depth  of  scum  

ft. 
.03 

ft. 
.00 

ft. 
.40 

ft. 
.00 

\ 

ft. 
00 

Depth  of  sludge         .    .    . 

75 

40 

30 

25 

50 

45 

Total  depth  of  scum  and  sludge 
Volume  of  scum  and  sludge  .... 

.78 
cu.  ft. 
15.0 

.40 

CM.  ft. 
3.84 

.70 
CM.  ft. 
16.8 

.25 

cu.ft. 
3.0 

.52 

CM.  ft. 

16.7 

.45 
cu.ft. 
7.2 

>These  measurements  were  taken  about  three  months  before  analysis  of  effluent  was  discon- 
tinued. The  tanks  were  put  into  operation  June  4  to  12,  1924,  and  measurements  made  September 
7,  1925.  Scum  and  sludge  measurements  were  continued  for  14  months  after  the  analysis  of  effluent 
was  discontinued;  the  results  of  the  latter  measurements  are  shown  in  Table  13  and  Fig.  22. 

TABLE  13. — SCUM  AND  SLUDGE  ACCUMULATION  FROM  FARM  SEWAGE  DURING  A 
PERIOD  OF  2  YEARS  8)4  MONTHS 


Chamber  No  

11 

12 

Jl 

J2 

Kl 

K2 

(8'x2.4'x5') 

(4'x2.4'xo') 

(8'x3'x4') 

(4'x3'x4') 

(8'x4'x3') 

(4'x4'x3') 

Volume  of  scum  and  sludge, 
cu.  ft  

20  4 

11.2 

40  3 

6.7 

32  3 

16.2 

Percentage  of  chamber  capacity 
occupied  by  scum  and  sludge 

21.2 

23.3 

42.0 

13.9 

33.6 

33.8 

from  two-chamber  tanks  of  equal  capacity  but  of  narrow-deep,  med- 
ium, and  wide-shallow  cross-section.  Table  9  outlines  the  conditions 
maintained  during  these  tests.  Information  on  the  functioning  of  the 
tanks  at  two  different  stages  of  their  operation  may  be  secured  from 
Tables  17  and  18.  In  this  instance  again  direct  comparisons  between 
the  tanks  treating  city  sewage  and  those  receiving  farm  sewage  should 
not  be  made,  since  those  treating  city  sewage  received  the  same  kind 
and  amount  of  sewage,  while  those  treating  farm  sewage  received  a 
sewage  flow  varying  in  quality  and  in  amount. 

Tank  temperature,  residue  on  evaporation,  and  settleable  solids 


328 


BULLETIN  Xo.  304 


[April, 


plotted  for  the  first  and  second  chambers  of  Tank  G,  are  shown  in 
Fig.  19.  Fig.  20  shows  similar  curves  for  the  first  and  second  cham- 
bers of  Tank  J.  Table  11  gives  the  results  of  scum  and  sludge 


Volume    of  scum  ajxt  sludge 
Volume    of 
Volume,    of  acv 


Mar     Apr         May       June        July         Au$         Sept 


FIG.  21. — RATE  OF  SCUM  AND  SLUDGE  ACCUMULATION  WITH  CITY  SEWAGE 

Comparing  the  first  and  second  chambers  of  each  tank,  the  scum  and 
sludge  accumulation  is  consistently  greater  in  the  first  chamber.  Little  difference 
in  rate  of  scum  and  sludge  accumulation  occurred  for  tanks  of  varying  depths. 
The  rate  of  accumulation  was  less  during  June  and  July  because  the  high  tank 
temperatures  resulted  in  a  more  complete  digestion  of  the  solids.  In  both  Studies 
II  and  III  the  scum  and  sludge  accumulations  were  greater  with  city  than  with 
farm  sewage. 


1928} 


EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS 


329 


measurements  made  on  tanks  treating  city  sewage,  and  Table  12  shows 
results  of  similar  measurements  made  on  tanks  treating  farm  sewage. 


¥  ^  go'SlJ^  £ 
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Vol 

Voluide 


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Chamber  It 


Ch&mber'lz 


Chamber  'J 


Chamber  Jz ' 


Chamber  Ki ' 


~ '  '"*"    ~  Chamber'At 


FIG.  22. — RATE  OF  SCUM  AND  SLUDGE  ACCUMULATION  WITH  FARM  SEWAGE 

Only  an  occasional  trace  of  scum  appeared  in  Chambers  12  and  K2,  hence 
the  one  dotted  line  for  these  chambers  represents  sludge  accumulation.  No 
measurements  were  taken  during  the  summer  of  1926,  but  it  is  probable  that 
high  temperatures  at  that  time  caused  a  somewhat  similar  reduction  of  scum 
and  sludge  to  that  observed  the  previous  summer.  The  tanks  were  put  in 
operation  in  June,  1924,  and  the  curves  show  the  accumulation  during  the  period 
of  2  years  8%  months. 


330 


BULLETIN  No.  304 


[April, 


A  number  of  curves  are  included  in  Figs.  21  and  22  which  show  rate 
of  scum  and  sludge  accumulation.  The  scum  and  sludge  accumulation 
in  Tanks  I,  J,  and  K  over  a  period  of  2  years  8%  months  are  shown  in 
Table  13,  and  Fig.  23  indicates  graphically  the  accumulation  in  Tank  J. 

Discussion  and  Conclusions 

A  study  of  the  data  collected  in  this  investigation  revealed  the 
following  facts  and  leads  to  the  conclusions  and  recommendations 
indicated : 

1.  No  definite  relationship  between  the  shape  of  cross-section  of  a 
septic  tank  and  the  efficiency  of  its  operation  was  discovered  (Table 


Inside  width  of  ta.nk-3-o" 

FIG.  23. — LONGITUDINAL  SECTION  OF  TANK  J 
The  black  areas  indicate  scum  and  sludge  accumulations  after 
2  years,  8%  months.  Data  on  the  scum  and  sludge  accumulation  of 
all  the  tanks  used  in  Study  III  are  shown  graphically  in  Figs.  21 
and  22. 

10) .  Since  the  effective  depth  gradually  becomes  less  as  scum  and  sludge 
accumulate,  it  would  seem  logical  to  select  a  cross-section  of  reason- 
able sewage  depth  (3%  to  4%  feet)  which  would  provide  the  required 
capacity  with  the  most  economical  construction  (considering  relative 
costs  of  floor,  walls,  reinforced  cover,  etc.),  and  at  the  same  time  pro- 
vide a  tank  length  which  would  be  satisfactory  for  the  settlement  of 
suspended  material. 

2.  The  effluent  from  the  second  chambers  of  all  tanks  (with  both 
city  and  farm  sewage)  showed  a  marked  improvement  over  the  effluent 
from  the  first  chambers  (Table  10) .  Similar  results  were  obtained  in 
the  investigation  of  single-  and  multiple-chamber  tanks.  Thus  in  both 
investigations  a  comparison  of  one-chamber  tanks  with  two-chamber 
tanks  of  50  percent  larger  capacity  shows  a  much  better  effluent  from 
the  two-chamber  tanks.  The  advantage  of  the  two-chamber  tanks  is 
probably  due  partly  to  additional  baffling  and  reduction  of  gassing 
near  the  outlets,  and  partly  to  the  additional  capacity  provided. 


1928}  EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS  331 

3.  The  narrow,  medium,  and  wide  tanks  treating  farm  sewage  had 
an  average  total  retention  period  of  273,  118,  and  148  hours  respect- 
ively (Table  9).    The  effluent  from  the  second  chamber  of  each  of 
these  tanks  showed  considerable  improvement  over  that  from  the 
first  chamber  (Table  10).    Considering  the  long  retention  periods  of 
these  tanks,  the  results  indicate  that  it  is  good  practice  to  allow  ample 
capacity  in  the  design  of  farm  septic  tanks.    Some  factors  to  be  con- 
sidered in  deciding  upon  the  capacity  of  the  tank  are:    quality  of 
effluent  desired,  cost  of  constructing  tank,  and  cost  of  maintenance. 
(The  cost  of  cleaning  would  be  less  for  a  large  tank  since  fewer  clean- 
ings are  required) . 

4.  The  curves  for  rate  of  scum  and  sludge  accumulation  (Figs.  21 
and  22)  show  in  gejneral  a  gradual  increase  in  combined  volume.   The 
decrease  during  June,  July,  and  August,  1925,  in  tanks  treating  farm 
sewage  (Fig.  22)  was  apparently  due  to  the  better  digestion  of  solids 
which  takes  place  during  periods  of  higher  tank  temperature,  and  also 
to  the  passing  out  of  more  solids  with  the  effluent  during  periods  of 
high  tank  temperature.     (During  the  previous  study  the  effluent  was 
high  in  solids  while  the  tank  temperature  was  high).    No  scum  and 
sludge  measurements  were  taken  during  the  summer  of  1926,  but  it  is 
probable  that  a  decrease  in  volume  occurred  similar  to  that  of  the 
previous  summer. 

5.  Curves  for  scum  and  sludge  (Fig.  22)  indicate  an  unloading 
of  sludge  from  Chamber  Kl   into  K2  during  December,   1926,  and 
January  and  February,  1927.    This  was  evidently  due  to  gassing  in 
Kl,  and  the  consequent  rising  of  sludge  from  the  bottom  of  the  cham- 
ber and  an  increase  in  scum  volume,  the  gassing  and  disturbance  in- 
creasing the  solids  carried  over  into  K2.   The  additional  chamber  was 
of  special  advantage  in  preventing  large  amounts  of  solids  from  pass- 
ing into  the  final  disposal  tile. 

6.  In  designs  for  septic  tanks  allowance  should  be  made  for  scum 
and  sludge  storage  in  order  that  efficient  operation  may  be  assured  for 
considerable  periods  without  cleaning.  The  volume  of  scum  and  sludge 
accumulation  during  a  period  of  2  years  8%  months  for  septic  tanks 
treating  sewage  from  three  different  farm  homes  is  given  in  Table  13. 
The  average  accumulation  per  tank  during  this  time  was  42.36  cubic 
feet,  an  equivalent  accumulation  of  3.35  cubic  feet  per  person  per  year. 
During  the  2  years  8%  months  of  operation  the  average  volume  of 
scum  and  sludge  in  the  first  chambers  of  the  three  tanks  was  31  cubic 
feet,  or  32.3  percent  of  the  capacity  of  the  chamber,  and  the  average 
volume  in  the  second  chambers  was  11.36  cubic  feet,  or  23.7  percent  of 
the  capacity  of  the  chamber.    These  chambers  were  larger  than  ordi- 
narily recommended.   With  tank  chambers  of  the  size  shown  in  Fig. 
25  a  larger  percentage  of  the  tank  capacity  would  be  occupied  by 
scum  and  sludge;  on  the  other  hand,  under  normal  conditions,  with  a 


332 


BULLETIN  No.  304 


[April, 


concrete  slab  and  earth-fill  covering  (instead  of  plank),  more  favor- 
able temperatures  should  exist  for  sludge  digestion.  With  a  two- 
chamber  tank  3  feet  wide,  having  a  4- foot  depth  of  sewage  and  cham- 
ber lengths  of  6  feet  and  3  feet,  a  similar  accumulation  of  31  cubic 
feet  would  occupy  43  percent  of  the  capacity  of  the  first  chamber,  and 
an  accumulation  of  11.36  cubic  feet- would  occupy  31.5  percent  of  the 
capacity  of  the  second  chamber. 

IV— RECOMMENDATIONS  FOR  THE  DESIGN  OF  SIMPLE 
FARM  SEPTIC  TANKS 

The  following  recommendations  for  the  design  of  farm  septic  tanks 
are  based  upon  the  results  of  the  three  foregoing  investigations: 

1.  Make  allowance  for  an  average  sewage  flow  from  different- 
sized  farm  homes  as  follows: 

7  people,  25  gallons  per  capita  per  day 

9  people,  23  gallons  per  capita  per  day 

12  people,  20  gallons  per  capita  per  day 

15  people,  18  gallons  per  capita  per  day 

2.  For  a  single-chamber  tank  provide  an  effective  retention  period 
of  48  hours,  with  an  allowance  of  50  percent  additional  capacity  for 
sludge  storage,  or  a  total  retention  period  of  72  hours  of  sewage  flow. 
(Allowance  is  made  for  sludge  storage  in  order  to  make  possible  longer 
service  without  cleaning  the  tank) . 

3.  For  a  more  efficient  plant  use  a  two-chamber  tank.  Provide  a 
retention  period  of  72  hours  in  the  first  chamber  (effective  retention 
period  of  48  hours,  with  a  50  percent  additonal  capacity  for  sludge 
storage)  and  an  additional  retention  period  of  36  hours  in  the  second 
chamber,  or  a  total  retention  period  of  108  hours. 

4.  Make  the  minimum-sized  tank  large  enough  for  7  people:  (a) 
in  order  to  maintain  ample  tank  dimensions  for  proper  settlement  of 
solids;   (b)  to  allow  for  additional  people  in  the  house;   (c)  because 
the  reduction  in  cost  is  small  for  tanks  under  this  suggested  minimum ; 
(d)  with  less  than  7  people,  the  additional  capacity  insures  more  effi- 
cient operation  and  less  frequent  cleaning. 


TABLE  14. — SUGGESTED  CAPACITY  AND  DIMENSIONS  FOR  SEPTIC  TANKS  TO  ACCOM- 
MODATE DIFFERENT  NUMBERS  OF  PEOPLE 


Number 
of 
people 

Sewage 
flow  per 
person 
per  day 

Capacity  required  in  first 
chamber  for  72-hour 
retention 

Effective 
cross-section 

Length  for 
1-chamber  tank 
or  for  first 
chamber  of 
2-chambertankl 

Length  for 
second  chamber 
of  2-chamber 
tank 

7 
9 
12 
15 

gals. 
25 
23 
20 
18 

gals. 
525 
620 
720 
810 

Clt.ft. 

70 
83 
96 
108 

3'x4' 
3'x4' 
3'x4' 
3'x4' 

6'-0" 
7'-0* 
8'-0" 
9'-0* 

3'-0" 
3'-6" 
4'-0* 
4'-6» 

»The  lengths  given  in  the  sixth  column  and  an  effective  cross-section  measuring  3'x4'  provide 
approximately  the  capacities  given  in  the  fourth  column. 


192S\ 


EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS 


333 


5.  Use  a  tank  cross-section  3  feet  wide  with  a  4-foot  depth  of 
sewage.    (This  is  suggested  as  an  economical  cross-section  for  tanks 
accommodating  7  to  15  people)  . 

6.  Use  the  data  given  in  Table  14  for  the  suggested  capacities, 
length  of  chambers,  etc.,  for  different  numbers  of  people. 

7.  Refer  to  Fig.  24  for  a  suggested  design  for  a  single-chamber 
septic  tank,  and  to  Fig.  25  for  a  two-chamber  tank  with  a  partition 
between  the  chambers  designed  to  retain  scum  and  sludge  in  the  first 
chamber. 

8.  A  single-chamber  tank  of  the  design  shown  in  Fig.  24  has  an 


Jf-ih- 


TA&LS-    OP 


1      PLAN 
3HOW/NO  COVEK.  REMOVED 


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coyer 


Gracfa 


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£  "o£e»v  bottom  o/s/a&. 


A 


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CROSS  -3£CT/Off 


FIG.  24. — SUGGESTED  DESIGN  FOR  A  SINGLE-CHAMBER  SEPTIC  TANK. 
This  single-chamber  tank  should  give  reasonably  good  service  where  the 
final  disposal  of  the  effluent  is  not  a  serious  problem. 

advantage  over  a  two-chamber  tank  of  the  design  shown  in  Fig.  25, 
in  lower  cost  of  construction,  but  the  two-chamber  tank  has  important 
advantages,  as  follows:  (a)  fewer  solids  pass  out  with  the  effluent; 
(b)  there  is  less  danger  of  clogging  the  final  disposal  tile;  (c)  the  tank 
will  operate  efficiently  for  a  longer  period  without  cleaning;  (d)  be- 
cause of  the  longer  retention  period  fewer  pathogenic  organisms  pass 
out  with  the  effluent.1 


Statement  based  on  results  reported  by  Rockefeller  Institute  of  Medical 
Research  regarding  the  life  of  typhoid  and  dysentery  bacilli  in  septic  tanks. 


334 


BULLETIN  No.  304 


[April, 


9.  Considering  the  above  advantages,  the  two-chamber  tank  is 
recommended  for  best  results  and  might  well  be  considered  for  all  con- 
ditions. However,  the  single-chamber  tank  should  give  reasonably 
good  results  where  the  final  disposal  of  the  effluent  is  not  a  serious 
problem.  This  would  generally  be  true  where  one  or  more  of  the  fol- 


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J  Koufte/rods.  C»nf*r-  of  1-0^3  j?  " 
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5SCT/ON  5HOIV/NG 
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FIG.  25. — SUGGESTED  DESIGN  FOR  A  TWO-CHAMBER  SEPTIC  TANK 
While  more  expensive  to  construct  than  the  one-chamber  tank,  the  above 
two-chamber  tank  has  important  advantages  and  might  well  be  considered  for 
all  conditions.  Fewer  solids  pass  out  with  the  effluent;  there  is  less  danger  of 
clogging  the  final  disposal  tile;  the  tank  will  operate  efficiently  for  a  longer 
period  without  cleaning;  and  because  of  the  longer  retention  period  fewer  path- 
ogenic organisms  will  pass  out  with  the  effluent. 

lowing  conditions  exist:  (1)  disposal  tile  located  in  porous,  well- 
drained  soil;  (2)  plenty  of  area  available  for  disposal  tile;  (3)  water 
supply  in  a  well-protected  location;  and  (4)  small  family  using  tank 
(two  to  four  persons). 


APPENDIX 


336 


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EFFICIENCY  AND  DESIGN  OF  FARM  SEPTIC  TANKS 


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